Conventional extrusion dies include multi-manifold and single-manifold flow channel designs, and are illustrated by end feed dies (U.S. Pat. No. 5,234,649 to Cloeren), T-Slot dies, coathanger dies, and multi-stage preland dies (U.S. Pat. Nos. 5,234,649 and 5,256,052 to Cloeren). The specific flow channel geometries are well described, and include a flow channel comprising a transverse flow-providing manifold spanning a width, and a downstream flow channel portion.
The manifold may have a decreasing or constant cross-sectional area, and may have a variety of cross-sectional shapes, including a generally circular cross-section (U.S. Pat. No. 4,285,655 to Matsubara). Prior art FIG. 2 of Cloeren '649 illustrates a generally tear drop manifold shape, and FIG. 3 (incorporated herein by reference) of U.S. Pat. No. 5,120,484 to Cloeren illustrates a generally extended manifold shape, and in particular a generally rectangular manifold shape.
Generally transverse to the main flow direction, coathanger dies have rectilinear or curvilinear (Matsubara '655) manifold boundaries. The downstream flow channel portion provides fluid communication between the manifold and the flow channel exit orifice, and includes a transverse flow restriction zone. A transverse flow restriction zone is designed to provide a prescribed uniform, or non-uniform, mass flow distribution across the flow channel width.
The transverse flow restriction zone comprises a flow restriction gap formed by opposing preland surfaces of the downstream flow channel portion. The surfaces forming the gap may be generally parallel, or oblique, to each other, or may comprise portions generally parallel or oblique to each other, generally transverse to the main flow direction.
The flow restriction gap may be, or may not be, adjustable, or comprise an adjustable portion. A transverse flow restriction zone including an adjustable restrictor bar that extends across the flow channel width, is generally illustrated by FIG. 3 of U.S. Pat. No. 3,940,221 to Nissel and by U.S. Pat. No. 4,372,739 to Vetter.
As illustrated by the hybrid coathanger die of FIG. 1, a known prior art extrusion die 1 may be formed by die bodies 2 and 3 (partially shown), and includes a flow channel 4 that begins at a flow channel inlet 26 and ends at a flow channel exit orifice 28. An arrow at the flow channel inlet indicates the main direction of mass flow through the extrusion die. For clarity, features including the body assembly fasteners, have been omitted.
With reference to FIG. 2, a transverse flow-providing manifold 5 of flow channel 4 spans a width from within a region 15 (generally indicated) of transverse flow initiation. The flow channel inlet feeds the manifold, which has rectilinear boundaries generally transverse to the main flow direction. A boundary 65 delineates the coathanger portion of the flow channel from a wing-like (commonly known as a “gull-wing manifold”) portion of the flow channel suitable for receiving an internal deckle.
Referring again to FIG. 1, the manifold is a generally tear drop-shaped manifold, and includes fillet radii R1, R2. As illustrated, fillet radii R1, R2 are spaced apart by a back wall 12 of the manifold, however when coterminous with each other, may join to form a radial back wall. Back wall 12 is the most upstream boundary of the manifold along the manifold width. With further reference to FIG. 1 and for purposes of the present invention, “H” means manifold height, “HT” means manifold tangent height, “R” means manifold fillet radius, “L” means manifold length, “LT” means manifold tangent length, and “LFC” means flow channel length.
As further illustrated by FIG. 2, the functional width of manifold 5 may be reduced by internal deckles 19 in the die flow channel. The deckles may be slidably disposed to different extents in the manifold from an end of the flow channel. An end 23 of the deckle terminates transverse mass flow within the manifold. Reference is also made to U.S. Pat. No. 5,505,609 to Cloeren et al and U.S. Pat. No. 5,451,357 to Cloeren for the use of internal deckles. Otherwise, the manifold structural width generally corresponds to width W of flow channel 4.
Referring again to FIG. 1, downstream flow channel 6 includes a preland channel comprising a transverse flow restriction zone 7, and includes exit channel surfaces 8. The transverse flow prescription zone comprises a flow restriction gap 25 formed by opposing preland surfaces 62,63 of downstream flow channel 6. As illustrated, flow restriction gap 25 has a constant dimension across the flow channel width; however, as described, flow restriction gaps that dimensionally change across the flow channel width, may also be useful.
With continued reference to FIG. 1, opposing surfaces 40,42 of the manifold have a constant angular relationship to each other (indicated as angle α), along the manifold width. Manifold surfaces 40,42 intersect preland surfaces 62,63 to form a boundary 34 (shown in FIG. 2) that includes termini 41,43, respectively. Exit edges 18 are formed by the intersection of exit channel surfaces 8 with die faces 9.
As described by U.S. Pat. No. 3,344,473 to Achterberg et al, a flow stream comprises flow stream lines. With continued reference to FIG. 2, and with reference also to FIGS. 1 and 2 (incorporated herein by reference) of U.S. Pat. No. 5,234,649 to Cloeren, as generally illustrated, laminar stream lines in the manifold flow along trajectories generally transversely from region 15 of transverse flow initiation, and differ with respect to a locus of transverse flow initiation and a locus substantially ending transverse flow within the manifold.
Continuing with particular reference to FIG. 2, transverse flow of stream lines generally begins proximate to the entrance to a transverse flow-providing manifold, and the streamlines flow transversely toward opposing ends 20 of the manifold. The transverse trajectory of a laminar stream line path, from a locus of transverse flow initiation of a stream line to a locus substantially ending its transverse flow in the manifold, is prescribed by the resistance to flow imposed upon the stream line by downstream channel portion 6, including the transverse flow restriction zone 7. Under steady state conditions, the stream line paths are fully developed and stationary. Accordingly, transverse flow of each stream line begins at its own locus of transverse flow initiation within the region of transverse flow initiation, and substantially ends within the manifold at its own respective locus, as generally illustrated by selected stream lines 13, 14, 16, 17.
Locus 13 is selected within region 15 along a stream line trajectory having minimal transverse displacement from the midline of the manifold width. Locus 14 is selected within region 15 along a second stream line, and locus 17 substantially ends transverse flow of the second stream line within the manifold proximate to termination of transverse mass flow in the manifold. Locus 16 substantially ends transverse flow within the manifold of a prescribed trajectory of a third stream line flowing from region 15, and is selected between locus 13 and locus 17.
Most all, if not all, thermoplastic polymers exhibit a time/temperature dependent rate of degradation according to which the higher the temperature, the higher the rate of degradation per unit time. Some thermoplastic polymers, such as for example polyvinyl chloride (PVC) have a high sensitivity to time/temperature dependent degradation; whereas other polymers, such as for example polypropylene (PP), exhibit relatively high tolerance to time/temperature dependent degradation.
Typically, the majority of the residence time for a mass flow passing through an extrusion die, is in the die manifold. Thus, for a highly time/temperature-sensitive fluid mass, minimization of the manifold residence time, and accordingly residence time within the die flow channel, is essential to minimize time/temperature dependent degradation.
For a given mass and given flow rate, a relatively smaller manifold cross-sectional area results in a relatively greater average mass flow velocity, and thereby provides an increase of the flow velocity along stream line paths, and the mass flow exchange rate is increased, and the average residence time is relatively less. By the term “mass flow exchange rate” is meant the frequency (rate/time) at which a mass unit is replaced by another mass unit.
Accordingly, in the prior art practice of extruding highly time/temperature-sensitive polymers, the manifold cross-sectional area is typically minimized in order to increase flow velocities along stream line paths and thereby increase the mass flow exchange rate and minimize the manifold residence time, particularly at an end region of the manifold where flow velocities and mass flow exchange rates are lowest and cumulative manifold residence time is the greatest. Increasing the mass flow exchange rate comes at the opposing expense of an increase in the manifold pressure drop. Thus, experienced artisans must balance the benefit of an increased mass flow exchange rate against an increased pressure drop to determine a preferred manifold design.
A problem associated with a change of the flow direction from a main flow direction to a generally transverse direction can occur within, and proximate to, the region of transverse flow initiation as a result of flow stream entrance affects (diagrammatically illustrated as “working energy loss” in FIGS. 1-2 of U.S. Pat. No. 5,234,649 to Cloeren). Such entrance affects may cause a region of flow transition stagnation, which is generally indicated and diagrammatically illustrated by generally arched lines in FIGS. 1 and 2. Transition stagnation is characterized by relatively low specific flow rates, or stagnant flow, and may result in deleterious time/temperature dependent polymer degradation. U.S. Pat. No. 3,344,473 to Achterberg et al. describes degradation of PVC mainly in an area corresponding to the regions of transverse flow initiation and of transition stagnation.
A prior art practice for reducing the time/temperature dependent degradation associated with transition stagnation, is to provide a compound angle (aka “PVC fly cut”) in the main flow direction, at the region of transverse flow initiation. This practice has been most commonly used for a coathanger-shaped flow channel.
Transition stagnation proximate to the region of transverse flow initiation, has also been reduced by elongation of the manifold by increasing the manifold length in the main flow direction. Although this solution is beneficial within and proximate to the region of transverse flow initiation, this solution may be contrary to optimizing the mass flow exchange rate and minimizing the residence time across the manifold width, and in particular in a manifold end region. As a result, degradation of a time/temperature-sensitive fluid mass can also result in a manifold end region.
With continued reference to FIGS. 1 and 2, a further problem with minimizing the manifold cross-sectional area is an increase in flow resistance which results in an increase in the pressure differential from region 15 of transverse flow initiation to ends 20 of manifold 5. An increase in pressure results in an increase of force, exerted generally perpendicular to the flow channel surfaces wetted by a fluid mass flowing through flow channel 4, and acting upon the unitary structure of the extrusion die. As a result, differential deflection of die bodies 2, 3 may occur. A die having an oblique relationship of a manifold upstream boundary 22 to the die exit orifice or having a differential wetted surface area across its width such as a coathanger die, is typically subject to more differential clamshell deflection, as compared to a die having a generally uniform wetted surface area provided by a manifold upstream boundary being generally parallel to the die exit orifice, as illustrated by FIGS. 10-13 of the Cloeren '649 patent and by FIGS. 4 and 5 of U.S. Pat. No. 5,494,429 to Wilson et al. Differential clamshell deflection of the unitary die structure negatively affects a prescribed mass flow distribution across the flow channel width.
Long felt needs of flow channel design include minimizing flow transition stagnation within and proximate to the region of transverse flow initiation, minimizing differential clamshell defection across the flow channel width, and improved streamlining of the mass flowing through the manifold, while beneficially reducing residence time in the transverse flow-providing manifold.